1. Field of the Invention
The present invention relates to a polymer electrolyte and a fuel cell employing the same, and more particularly, to a polymer electrolyte that has a high ionic conductivity at high temperatures and a fuel cell employing the same.
2. Discussion of the Background
Fuel cells may be classified according to the electrolyte used in the fuel cell. The types of fuel cells include polymer electrolyte membrane fuel cells (PEMFCs), phosphoric acid fuel cells (PAFCs), molten carbonate fuel cells (MCFCs), and solid oxide fuel cells (SOFCs). The working temperatures of the fuel cells and their constituent materials vary depending on the type of electrolyte used in a cell.
The basic PEMFC includes an anode (fuel electrode), a cathode (oxidizing agent electrode), and a polymer electrolyte membrane interposed between the anode and the cathode. The anode includes a catalyst layer to promote the oxidation of a fuel and the cathode includes a catalyst layer to promote the reduction of an oxidizing agent.
Examples of fuel that may be supplied to the anode include hydrogen, a hydrogen-containing gas, a mixture of methanol vapor and water vapor, and an aqueous methanol solution. Examples of the oxidizing agent supplied to the cathode include oxygen, oxygen containing gas, and air.
In the anode of the PEMFC, fuel is oxidized to produce protons and electrons. The protons migrate to the cathode through an electrolyte membrane and the electrons migrate to an external circuit (load) through a conductive wire (or current collector). In the cathode of the PEMFC, oxygen reacts with the migrated protons and electrons, which are supplied from the external circuit through another conductive wire (or current collector), to produce water. The migration of electrons from the anode to the cathode via the external circuit provides electrical power.
The polymer electrolyte membrane acts as an ionic conductor for the migration of protons from the anode to the cathode and also acts as a separator for preventing contact between the anode and the cathode. Therefore, the polymer electrolyte membrane properties should include sufficient ionic conductivity, electrochemical safety properties, high mechanical strength, thermal stability at the operating temperature of the fuel cell, and should be easily formed into a thin layer.
A conventional polymer electrolyte membrane may be composed of a sulfonated perfluorinated polymer that has fluorinated alkylene in the backbone and sulfonic acid groups at the terminal position of fluorinated vinylether side chains, such as NAFION manufactured by Dupont. This type of polymer electrolyte membrane should contain a proper amount of water to have sufficient ionic conductivity.
However, a conventional polymer electrolyte membrane has a seriously lowered ionic conductivity at operating temperatures of about 100° C. or higher due to loss of moisture by evaporation, and at temperatures of about 100° C. or higher, it may not act as an ionic conductor. Thus, it is difficult to operate a PEMFC using a conventional polymer electrolyte membrane under atmospheric pressure at about 100° C. or higher. Conventional PEMFCs have been operated at about 100° C. or lower, for example, at about 80° C.
Various methods have been proposed to raise the operating temperature of a PEMFC to 100° C. or higher, including a method providing a PEMFC with a humidification apparatus, a method operating a PEMFC under pressurized conditions, and a method using a polymer electrolyte that does not require humidification.
When a PEMFC is operated under pressurized conditions, the boiling point of water increases, which can raise the operating temperature. Additionally, the use of a pressurizing system or a humidification apparatus increases the size and weight of the PEMFC and reduces the efficiency of the power generating system. Therefore, a need exists for a polymer electrolyte membrane that has a high ionic conductivity even at a relative humidity of about 0%.
Examples of polymer electrolyte membranes that do not require humidity include polybenzoimidazole (PBI), and polybenzoimidazole doped with sulfuric acid or phosphoric acid, which are disclosed in Japanese Patent Laid-Open Publication No. Hei 11-503262.
The thermal and chemical stability of an electrolyte membrane composed of PBI is good at high temperatures, but phosphoric acid impregnated in the electrolyte may leak from the cell when the cell is used for long periods.
U.S. published patent application 20040131910A discloses a method of preparing sulfonated polyether ketone ketone using a diphenyl ether and a benzenedicarboxylic acid derivative and use of the sulfonated polyether ketone ketone prepared thereby as a membrane for fuel cells.